Sorry, I guess I was not clear. The brass is sections of thin stamped sheet wrapped around a wooden framework and nailed in place. The mirror itself is regular glass mirror. I will take a picture tomorrow. There was an antiqued dark brown patina in the lowest parts of the design. Since it is not an antique, this is done with patina chemicals. I think there was a thin and incomplete coat of varnish or lacquer on it, which was why it was so hard to clean. On the parts where I had to scrub really hard to remove the green crusty corrosion or the red or brown discoloration under that, all this brown patina was removed. After cleaning, washing, and drying, I applied NEW patina chemical from a bottle. I keep this for my copper sculptures. I have only made things from new metal and have never had to deal with corrosion like this before. I have cleaned some of my old thrift store tarnished copper/brass stamped plates before, and when the discoloration was gone it stayed gone (then I lacquered them). But I have never before had to deal with corrosion that popped up again right after cleaning.

I pulled out ye olde detergent formulation handbook (yes, I have one)... I see that sodium metasilicate (powder) and sodium metasilicate pentahydrate (liquid) "provide alkalinity and can protect soft metals such as aluminum, copper, zinc, brass and tin from the corrosive effects of strongly alkaline cleaning solutions. Silicates also reduce the redeposition of soils." My book also states that STPP is often combined with sodium metasilicate to produce a synergistic effect where the two work better together. Also of note regarding STPP, it cannot solubilize fats on its own, but can with the addition of SDDBS (sodium dodecyl benzene sulfonate). Some people in this forum are under the impression that STPP can rid your machine of gunk and clean oil stains from clothing, which doesn't appear to be the case. However, my book does recommend that if you are using detergents with a precipitating softener (sodium carbonate, soda ash, wash soda, etc) in hard water, adding STPP will prevent those precipitates from agglomerating onto your washer parts. You will note that the vast majority of drum spiders in front loaders are made from aluminum, a metal that can be corroded by a strongly alkaline cleaning solution. The addition of silicates to such a solution should minimize the corrosive effects on the spider assembly.

Need more information. You say the beam will be recessed into the ceiling. Is the ceiling coming down or the beam going up? Is what is shown in the picture a wood beam, a decorative wood box or a wood box surrounding a metal beam. If it's the latter you will be limited by how far above the ceiling the beam ends up.

Did you use the rubber gasket included and required for this connection? If so it is possible it wasn't tightened enough and many times shoddy ground threads of the heater nipple were not straight and perhaps the heat has loosened the connection.